A semiconductor device fabricating process includes a deposition process for forming a thin film on a wafer and an etch process for forming a fine circuit pattern on the thin film. These processes are repeatedly carried out until a desired circuit pattern is formed on the wafer. Following formation of the circuit pattern, a large number of windings are formed at the surface of the wafer. With the recent trend toward finer semiconductor devices, the structure of semiconductor devices is multi-layered and the number of windings formed at the wafer surface and a step difference therebetween are increasing. If the wafer surface is not planarized, problems such as defocus occur during a photolithographic process. Thus, the wafer surface must periodically be polished so as to be planarized.
A variety of surface planarizing techniques have been developed to planarize a wafer surface. Particularly, a chemical mechanical polishing (CMP) apparatus is widely used due to the superior planarity that can be obtained for a narrow area as well as a wide area.
The CMP apparatus chemically mechanically polishes a wafer surface coated with tungsten or oxide and can achieve a very fine polishing. Mechanical polishing is performed by rotating a wafer that is pressed against a polishing pad, so there is a frictional force between the polishing pad and the wafer surface to polish the wafer surface. Chemical polishing is performed by polishing a wafer surface by feeding slurry, which is a chemical abrasive agent, between a polishing pad and a wafer.
Referring to FIG. 1, a conventional chemical mechanical polishing (CMP) apparatus has a platen 120 to which a polishing pad 140 is attached, and a polishing head 160 is disposed over the platen 120. A wafer is mounted on the polishing head 160 such that a polishing surface is disposed against a polishing pad. The polishing head 160 applies a controllable pressure to a rear surface of a wafer to polish the wafer.
According to the above-described CMP apparatus, an entire surface of the wafer can regularly be polished while the amount of polishing a partial surface of the wafer cannot be controlled. Therefore, in a case where a wafer surface is winded because the deposition thickness on each part of the wafer is different, the windings are left even after the polishing process. That is, the wafer is not uniformly planarized.
Generally, a diameter of the polishing pad 140 is at least two times larger than that of a wafer, and the wafer rotates on the axis of the polishing pad 140. Thus, as the wafer diameter increases from 200 mm to 300 mm, the diameter of the polishing pad 140 becomes larger, thereby increasing manufacturing costs because a larger size polishing pad is required.
Therefore, a need exists for a chemical mechanical polishing apparatus that uniformly planarizes a semiconductor wafer having a deposition of varying thickness on each portion of a wafer and that accommodates larger size semiconductors wafers without increasing the size of the polishing pad required to planarized a surface of a semiconductor wafer.